1. Field of the Invention
This invention relates to the field of copolymer latices and, more particularly, to such latices having high solids content and low viscosity and being prepared from a mixture of copolymerizable monomers including vinyl acetate and an acrylate ester, methacrylate ester and/or vinyl ester of an alpha-branched saturated monocarboxylic acid.
2. Description of the Prior Art
Numerous procedures for the emulsion polymerization and copolymerization of various ethylenically unsaturated monomers, vinyl esters, acrylate esters and methacrylate esters among them, have been known for some time. In general, a latex derived from a vinyl ester and one or more other monomers copolymerizable therewith is prepared by first charging an aqueous phase containing water, surface active agent, buffer, catalyst or catalyst system of the free radical type and usually a protective colloid such as polyvinyl alcohol (PVA), to a reactor. The monomers are thereafter charged to the reactor either as separate streams with mixing occurring in the reactor or in admixture, and either all at once or incrementally, and following heating to polymerization temperature, polymerization is thereafter permitted to proceed, accompanied by constant agitation, substantially to completion. The resulting latex is cooled and filtered and can be used in many applications, e.g., as paints and other surface coating compositions, adhesives and textile treating agents.
Emulsion polymerization procedures featuring one or more aspects of the aforedescribed method of preparing copolymer latices are described in U.S. Pat. Nos. 2,496,384; 2,520,959; 3,248,356; 3,404,114; 3,423,353; 3,455,887; 3,483,171; 3,485,808; 3,547,857; 3,562,229; 3,804,881; and 4,039,500. Similar emulsion polymerization processes are described in Great Britain Pat. Nos. 1,133,867; 1,178,016; 1,191,649; 1,363,432; and 1,450,599. U.S. Pat. No. 3,423,353 to Levine, et al. in particular describes latices obtained from vinyl acetate and one or more other comonomers including alkyl acrylates and methacrylates such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate and n-propyl methacrylate, the latices being said to possess higher solids contents than obtainable using conventional emulsion polymerization techniques, while at the same time having reduced viscosities and improved freeze-thaw satbility. The latices are prepared in the conventional manner and once polymerization is substantially complete, a relatively large amount of water-soluble polymerization catalyst or intiator is added to the emulsion to reduce viscosity and increase particle size. It is speculated by patentees that this post-addition of catalyst results in some degradation of the surface active agent and the ether linkage-containing protective colloid when used, reducing their molecular weights by an oxidative mechanism which involves repture of the ether linkages. This oxidative degradation is thought to reduce the effectiveness of the surface active agent and protective colloid resulting in some agglomeration or particles. The larger average particle size of the emulsion is believed to be also responsible for the reduction in viscosity.